It is known to acquire an image of a scene by means of an image sensor comprising a plurality of identical or similar pixels (or elementary cells), for example arranged in an array of rows and columns. Each pixel for example comprises a photodiode having its capacitance discharged by a photocurrent according to a received light intensity. Conventionally, during an image acquisition phase, each pixel is first reset by recharging of its photodiode to a reset voltage. The photodiode is then isolated from the node of application of the reset voltage, and the voltage across the photodiode decreases according to the light intensity received by the pixel. After a predetermined integration period identical for all the sensor pixels, each pixel delivers an analog output signal representative of the voltage across its photodiode. This signal is digitized by an analog-to-digital conversion circuit, and forms the output value of the pixel. The output values of the sensor pixels altogether form an image of the scene.
To overcome certain disadvantages of conventional image sensors, especially in terms of dynamic range, of signal-to-noise ratio, and/or of power consumption, time code image sensors have been provided, where each pixel comprises a comparator comparing the voltage across the photodiode with a reference voltage. During an image acquisition phase, each pixel is first reset by recharging of its photodiode to a reset voltage. The photodiode of each pixel is then isolated from the node of application of the reset voltage, and the voltage across the photodiode varies according to the light intensity received by the pixel. When the voltage across the photodiode of a pixel reaches the reference voltage, the comparator output switches state. The pixel is said to turn on. The pixel then signals its position to a readout circuit external to the pixel, according to an asynchronous communication protocol of “handshake” type, comprising the transmission, by the pixel, of an address readout request signal addressed to the readout circuit, followed by the transmission, by the readout circuit, of an acknowledgement signal addressed to the pixel, and then by the transmission, by the pixel, of an address indication signal addressed to the readout circuit. The time of reading of the pixel address by the readout circuit is stored and forms the pixel output value. When a plurality of pixels simultaneously turn on, an arbitration may be performed by the readout circuit, to set the pixel readout order. A disadvantage of this type of sensors is linked to the complexity and to the bulk of the asynchronous communication circuits provided to enable each pixel, when it turns on, to signal its position to the readout circuit. Further, due to the use of a “handshake”-type asynchronous communication protocol, variable delays may occur between the effective turn-on time of a pixel and the time of reception of the pixel address by the readout circuit. This may cause parasitic artifacts in the image and/or information losses, for example, when a high number of pixels of the sensor turn on simultaneously or within a short time interval.